When forming fine patterns of a semiconductor integrated circuit, liquid crystal display, or other electronic device, the method of reducing and transferring by exposure the patterns of a photomask (also called a “reticle”) enlarging the patterns to be transferred 4- to 5-fold or so to a wafer or other substrate to be exposed using a projection exposure apparatus has been used.
The projection exposure apparatuses used for transfer have been shifting in exposure wavelength to the shorter exposure wavelength side so as to deal with the increasing fineness of semiconductor integrated circuits. At the present time, the mainstream wavelength is the 248 nm of KrF excimer laser, but the shorter wavelength 193 nm of ArF excimer laser is entering the commercial stage as well. Further, projection exposure apparatuses using shorter wavelength 157 nm of F2 laser or wavelength 126 nm of Ar2 laser or other light sources of the wavelength band called the “vacuum ultraviolet region” are also being developed.
A light beam of a wavelength of this vacuum ultraviolet region has an extremely large absorption due to oxygen or water vapor, hydrocarbon gas, etc. (hereinafter referred to as an “absorption gas”), so it is necessary to purge the light path through which exposure light passes of the oxygen or other absorption gas with a low absorption nitrogen or rare gas or other gas (hereinafter referred to as a “low absorption gas”).
For example, regarding the concentration of oxygen or water vapor, it is necessary to keep the average concentration in the light path to not more than the ppm order. If the standard of the residual concentration of the absorption gas does not meet the above standard, the exposure energy on the wafer or other exposed substrate will become remarkably low.
Note that the glass material passing this vacuum ultraviolet light is limited to fluorite glass etc. Therefore, in a refraction optical system, correction of color aberration is difficult. As a projection optical system, an optical system comprised of reflection mirrors (concave mirrors) and lenses, that is, a catiodioptic optical system, is employed. In this optical system, flat mirrors are also necessary for separating light beams incident on the concave mirrors and light beams reflected from the concave mirrors.
These reflection mirrors or flat mirrors are required to have a high reflectance with respect to light beams in large ranges of incident angles, so employment of reflection mirrors or flat mirrors made by coatings including metal layers are considered promising.
In this wavelength region, as a metal layer having a high reflectance, aluminum etc. may be mentioned, but as explained above, there is the problem that if the gas-purged light path has even a little oxygen or water vapor remaining it in, the photochemical reaction caused by the irradiation of the vacuum ultraviolet light will cause this aluminum layer to be oxidized resulting in its reflectance sharply dropping.
Oxidation is not a problem that occurs only for mirrors including aluminum layers and is a problem which similarly occurs even if using another metal. Further, not only reflection mirrors and flat mirrors, but also antireflection coatings formed on the lens surfaces oxidize by the photochemical reaction with residual oxygen or water vapor and fall in transmittance.
Reduction of the residual oxygen and water vapor concentration in the purge gas is important in raising the transmittance of the exposure light passing through the light path. In particular, regarding water vapor, moisture adsorbed at the barrels or lens holding mechanisms forming the optical system continues to disassociate slowly over a long time, so it is difficult to suppress the concentration to not more than 1 ppm.
If the reflectance of the reflection mirrors or the transmittance of the antireflection coatings of the lens surfaces falls, the exposure energy reaching the wafer or other substrate to be exposed falls, so there is the problem that the processing ability of the exposure apparatus falls and the initial performance of the exposure apparatus can no longer be maintained.
Further, even if the above oxidation does not occur, it is not possible to produce mirrors with that high a reflectance in the vacuum ultraviolet region. The upper limit of the reflectance is not more than 90% or so. Therefore, part of the exposure light is absorbed by the mirrors and make the mirrors pick up heat. As a result, the mirrors are liable to deform due to heat expansion.